Introduction to strongSwan » History » Version 24

Noel Kuntze, 09.03.2017 14:06
routes, route and policy based IPsec

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{{title(Introduction to strongSwan)}}
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h1. Introduction to strongSwan
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This document is intended to give an introduction to strongSwan for new users (or existing users with catching-up to do).
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h2. Prerequisites
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It is assumed that the reader of this document...
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* understands the principles of networking, from setting IP addresses and DNS servers to basic firewalling
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* grasps the basic concepts of authentication based on public-key cryptography and a public-key infrastructure (PKI)
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* knows how to install binary software packages or how to compile source code following instructions
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* is adept on the console
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If you don't have such knowledge, there exist many ready-to-use appliances that provide remote IPsec access.
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One such device is the "revobox": by "revosec": that is based on strongSwan.
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h2. Securing a Network
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strongSwan is a complete IPsec solution providing encryption and authentication to servers and clients.
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It can be used to secure communications with remote networks, so that connecting remotely is the same as
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connecting locally.
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* *Gateway*: The gateway is usually your firewall, but this can be any host within your network.
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Often the gateway is also able to serve a small network with DHCP and DNS.
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In the image above the hosts *moon* and *sun* serve as gateways for the internal hosts *alice* and *bob*,
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* *Remote access / Roadwarrior clients*: Usually, roadwarriors are laptops and other mobile devices
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connecting from remote to your network using the gateway. In the image above *carol* represents a
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roadwarrior who wants to access either of the two networks behind the two gateways.
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* *Remote hosts / Host-to-Host*: This can be a remote web server or a backup system. This is illustrated
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in the image by host *winnetou* and either of the gateways. The connection between the two hosts can
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usually be initiated by either one of them.
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* *Remote sites / Site-to-Site*: Hosts in two or more subnets at different locations should be able to access
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each other. Again referring to the image above, the two subnets @ and @ behind
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gateways *moon* and *sun*, respectively, might be connected, so that the hosts *alice* and *bob* may securely
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communicate with one another.
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On our website you'll find dozens of [[ConfigurationExamples|complete configuration examples]] covering these and similar situations.
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Better and quickly applicable configurations can be found at [[SaneExamples]].
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h2. IKE and IPsec Basics
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*strongSwan is basically a keying daemon*, which uses the "Internet Key Exchange": protocols (IKEv1 and IKEv2)
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to establish _security associations_ (SA) between two peers. IKE provides strong authentication of both peers and derives
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unique cryptographic session keys.  Such an IKE session is often denoted *IKE_SA* in our documentation.
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Besides authentication and key material IKE also provides the means to exchange configuration information and
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to negotiate IPsec SAs, which are often called *CHILD_SAs*.  IPsec SAs define which network traffic is to be secured
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and how it has to be encrypted and authenticated.
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A CHILD_SA consists of two components:
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# The actual IPsec SA describing the algorithms and keys used to encrypt and authenticate the traffic
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# The policies that define which network traffic shall use such an SA.
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The policies work both ways, that is, only traffic matching an inbound policy will be allowed after decryption. Policies are derived from the
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traffic selectors (TS) negotiated via IKE when establishing a CHILD_SA.
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Unprotected traffic that the kernel receives for which there is a matching inbound IPsec policy will be dropped. This is a security feature.
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*strongSwan installs the negotiated IPsec SAs and SPs into the kernel by using a platform dependent kernel API.*
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The actual *IPsec traffic is not handled by strongSwan* but instead by the network and IPsec stack of the operating
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system kernel.
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The mentioned distinction between policies and SAs often leads to *misconceptions*.  For instance, referring to the image
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above, if host moon has a site-to-site tunnel to host sun (connecting the two networks and,
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and host carol has a roadwarrior connection to host sun (from which carol received a virtual IP address of,
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then carol wont be able to automatically communicate with alice, even if forwarding is enabled on sun.  This is because
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there is no IPsec policy allowing traffic between carol ( and alice ( An additional SA between moon
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and sun, connecting the virtual subnet with would be a possible solution to this issue.
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Generally, IPsec processing and routing are two different topics. IPsec is often just bumped into the stack (policy based)
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and the original routing decision for the unprotected packet also applies to the protected packet.
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This is the reason charon, by default, installs specific routes to the remote part of the TS (in newer versions of charon (>5.5.0),
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routes aren't installed for transport mode CHILD_SAs).
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An exception to this is route based IPsec, which uses interfaces to control what packets are going to be
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processed by each tunnel to a unique participant. Route based IPsec is less flexible
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than policy based IPsec.
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[[IPsecDocumentation|More information about IPsec and IKE]] can be found on our wiki.
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h2. Authentication Basics
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To ensure that the peer with which an IKE_SA is established is really who it claims to be it has to be authenticated.
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strongSwan provides several methods to do this:
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* *Public Key Authentication*: This uses *RSA or ECDSA X.509 certificates* to verify the authenticity of the peer.
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** Certificates can be self-signed, in which case they have to be installed on all peers, or signed by a common
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Certificate Authority (CA). The latter simplifies deployment and configuration a lot as the gateway only
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needs the CA certificate to authenticate all peers that provide a valid certificate signed by that CA.
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** "Certificate Revocation Lists": (CRLs) or the "Online Certificate Status Protocol": (OCSP) may be
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used to verify the validity of certificates.
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** To securely store private keys [[SmartCards|smart cards]] may be used via the [[PKCS11Plugin|PKCS#11 plugin]].
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** In order to prevent "man-in-the-middle attacks": the identity claimed by the peer has to be confirmed by
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the certificate, either by the _subject_ or a _subjectAltName_ extension.
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* *Pre-Shared-Key (PSK)*: A pre-shared-key is an easy to deploy option but it *requires strong secrets* to be secure.
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** If the PSK is known to many users (which is often the case with IKEv1 XAuth with PSK) any user who knows
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the secret could impersonate the gateway. Therefore this method is *not recommended* for large scale
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* *Extensible Authentication Protocol (EAP)*: This covers several possible authentication methods, some are
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based on username/password authentication (EAP-MD5, EAP-MSCHAPv2, EAP-GTC) or on certificates (EAP-TLS),
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some can even tunnel other EAP methods (EAP-TTLS, EAP-PEAP).
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** The actual authentication of users may be delegated to a RADIUS server with the [[EAPRAdius|eap-radius plugin]].
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** EAP authentication can only be used with IKEv2 and for some methods with IKEv1 using the [[XAuthEap|xauth-eap plugin]].
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* *eXtended Authentication (XAuth)*: XAuth provides a flexible authentication framework within IKEv1. It is mainly
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used for username/password based authentication. Also, it is generally used as a second authentication method
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after a mutual authentication with either certificates or PSK. With _IKEv1 hybrid authentication_ is is, however,
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possible to authenticate the gateway with a certificate and use only XAuth to authenticate the client.
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With IKEv2 it is possible to use multiple authentication rounds, for instance, to first authenticate the "machines" with
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certificates and then the "user" with an username/password-based authentication scheme (e.g EAP-MSCHAPv2).  It is also
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possible to use asymmetric authentication, for instance, by authenticating the gateway with a certificate and the client
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with an username/password-based EAP method (in the first authentication round).
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Again, our website provides dozens of [[ConfigurationExamples|configuration examples]] covering these and other authentication options.
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h2. Configuration Files
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The configuration files used by strongSwan are as follows:
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* [[IpsecConf|ipsec.conf]]: provides the configuration of IPsec connections
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* [[IpsecSecrets|ipsec.secrets]]: lists the secrets (pre-shared keys, private keys)
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* [[IpsecDirectory|ipsec.d]]: stores certificates and private keys
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* [[strongswanConf|strongswan.conf]]: allows one to configure global settings
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h3. Terminology
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*left* and *right* as used in the [[ipsec.conf]] file denote the two endpoints of an IKE_SA:
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* *left* means the *local* peer, i.e. the one on which the config file is stored
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* *right* then is the *remote* peer
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You can easily remember this by looking at the first letter of the two terms (*left=local*, *right=remote*).
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h3. Other Configuration Sources
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Since version:5.2.0 the [[vici]] plugin provides a new configuration backend. The configuration is stored in [[swanctl.conf]]
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and the [[SwanctlDirectory|swanctl directory]].
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The configuration may also be loaded from [[SQL|an SQL database]] or provided by custom plugins like the one used with
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the [[NetworkManager|NetworkManager plugin]].
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h2. Installation
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The [[InstallationDocumentation|installation of strongSwan]] is covered in a separate part of the wiki.
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Using binary packages provided by your distribution is generally recommended as it makes maintenance easier.
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Unfortunately, this means that you are often not able to use the most recent version.
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h2. Invocation and Maintenance
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strongSwan is usually controlled with the [[IpsecCommand|ipsec command]]. @ipsec start@ will start the [[IpsecStarter|starter daemon]] which in turn
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starts and configures the keying daemon [[charon]].
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Connections defined as [[ConnSection|conn sections in ipsec.conf]] can be started on three different occasions:
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* *On startup*: Connections configured with _auto=start_ will automatically be established when the daemon is started. 
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They are not automatically restarted when they go down for some reason. You need to specify other configuration settings 
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(_dpdaction_ and/or _closeaction_) to restart them automatically, but even then, the setup is not bullet-proof and will 
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potentially leak packets. You are encouraged to use _auto=route_ and read the [[SecurityRecommendations|SecurityRecommendations]] to take care of any problems.
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* *On traffic*: If _auto=route_ is used, IPsec policies for the configured traffic (_left|rightsubnet_) will be installed and traffic
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matching these policies will trigger events that cause the daemon to establish the connection.
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* *Manually*: A connection that uses _auto=add_ has to be established manually with @ipsec up <name>@ or by a peer.
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It is also possible to use @ipsec route <name>@ to install policies manually for such connections, like _auto=route_ would do it on startup.
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After an SA has been established @ipsec down@ may be used to tear down the IKE_SA or individual CHILD_SAs.
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Whenever the [[ipsec.conf]] file is changed it may be reloaded with @ipsec update@ or @ipsec reload@. Already established
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connections are not affected by these commands, if that is required @ipsec restart@ must be used.
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If [[ipsec.secrets]] or the files in [[IpsecDirectory|ipsec.d]] have been changed the [[IpsecCommand#Reread-Commands|ipsec reread...]] commands may be used to reload these files.
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End-entity certificates placed in [[IpsecDirectoryCerts|ipsec.d/certs]] are not reloaded automatically, instead they are loaded whenever referenced
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with _left|rightcert_ in a [[ConnSection|conn section]]. Using the [[IpsecCommand#Purge-Commands|ipsec purge...]] commands may be required in order for the new files to be used.
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Using the [[Ipseccommand#List-Commands|ipsec list...]] commands will provide information about loaded or cached certificates, supported algorithms and
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loaded plugins.
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h2. Logging and Monitoring
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If you run into problems, increasing the log level might help you understand what exactly went wrong. The different
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[[Loggerconfiguration|logging options]] are described on our wiki and the [[strongswan.conf]] man page.
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Whenever you encounter a log message similar to @"received ... error notify"@, where @...@ is, for instance,
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@NO_PROPOSAL_CHOSEN@ or @TS_UNACCEPTABLE@, you should consult the logs of the *remote peer* so as to find out why
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it generated that error notify in the first place.
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Starting the daemon with @ipsec start --nofork@ prevents it from forking and will log directly to the console (in case
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loggers are configured in [[strongswan.conf]] make sure one of them logs to _stderr_ or _stdout_).
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The [[IpsecCommand|ipsec]] @status@ and @statusall@ commands will provide information about the established and configured connections.
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On Linux the _iproute2_ package provides the @ip xfrm state@ and @ip xfrm policy@ commands to request detailed
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information about the IPsec SAs and policies installed in the kernel.  Adding the @-s@ option will display extensive
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statistical information like the number of transmitted or invalid packages. On other platforms the _setkey_ command
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from the _ipsec-tools_ package provides similar information.
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_tcpdump_ and _wireshark_ are also often useful to debug problems.
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When testing a connection with _ping_ make sure to select a source IP address (with the @-I@ option) that is included in the
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local traffic selector (also see [[IntroductionToStrongswan#Site-to-Site-Configurations|Site-to-Site Configurations]] below).
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h2. PKI
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To use certificate based authentication you'll need to create either self-signed certificates or setup a whole public-key
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infrastructure (PKI), consisting of a Certificate Authority (CA), optional intermediate CAs and end-entity certificates plus
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certificate revocation lists (CRL) or other methods like OCSP to verify the validity of certificates.
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One of the easiest ways to generate certificates is to use the [[IPsecPki|ipsec pki]] utility. Since setting up a whole PKI can be quite
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complex, we only [[SimpleCA|provide instructions]] to get you started.
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OpenSSL is also a widespread alternative to generate certificates, as are several GUI based [[CAmanagementGUIs|CA management utilities]].
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Commercial CA management tools like Microsoft's are also often used for large scale CAs.
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h2. Routing
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On Linux, strongSwan installs routes into routing table 220 by default and hence requires the kernel to support policy
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based routing.
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You can make the daemon install the routes into any table you like, or you can disable it completely.
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For those purposes, the _charon.install_routes_, _charon.routing_table_ and _charon.routing_table_prio_ settings
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in [[strongswan.conf]] may be used. When a tunnel is established between two subnets, charon tries to find local IPs in the tunneled
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local subnets. Such an IP must be configurd with _scope global_ to be viable for the lookup. If a valid IP is found, charon will install
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a route pointing to the remote subnet where the source IP is set to the found IP. This results in routes like the following:
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In that example, the local IP would be The remote subnet would be
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This is done so packets to the remote subnet are sent with the correct source IP, so the IPsec policies match and traffic
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from the local machine to the remote subnet will be secured with IPsec.
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To avoid conflicts with these routes (especially if [[VirtualIP|virtual IPs]] are used), the _kernel-netlink_ plugin manually parses the
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host's routing tables to determine a suitable source address when sending IKE packets.  On hosts with a (very) high number
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of routes this is quite inefficient. In that case, setting _charon.plugins.kernel-netlink.fwmark_ in [[strongswan.conf]] is
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recommended as it will allow using a more efficient source address lookup.
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In order to detect connectivity changes strongSwan parses the events that the kernel sends when a route is installed or
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deleted and hence could cause high CPU load when you run it on a system that receives a lot of routes via dynamic
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routing, for example. You can disable it using the _charon.process_route_ setting in [[strongswan.conf]].
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If IPv6 is used then make sure to [[IPv6NDP|bypass NDP(Neighbor Discovery Protocol) traffic]] if necessary.
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It is possible that you encounter MSS/MTU problems when tunneling traffic. Please refer to [[ForwardingAndSplitTunneling#MTUMSS-issues|Forwarding and Split-Tunneling]]
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for details.
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h2. Remote Access Configurations
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In this section we present example configurations for common remote access use cases.  In these so called _roadwarrior_
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scenarios mobile clients will be able to connect to a remote network.
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Because these clients most likely connect from unknown IP addresses the gateway will use _right=%any_ to literally accept
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connections from anywhere.  To simplify [[ForwardingAndSplitTunneling|routing traffic back]] to the clients and because roadwarriors are often located
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behind one or more NAT devices, the use of [[VirtualIP|virtual IP addresses]] is necessary.
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The virtual IPs can either be from a distinct subnet or actually from the subnet behind the gateway (by use of the
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[[farpplugin|farp]] and optionally the [[dhcpplugin|dhcp]] plugins).
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Whether roadwarriors will send all traffic to the gateway or use [[ForwardingAndSplitTunneling|split-tunneling]], that is, only send traffic for specific
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destinations through the tunnel, is also something to consider.  It is explained more detailed in [[ForwardingAndSplitTunneling|Forwarding and]]
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The above page also explains how traffic is [[ForwardingAndSplitTunneling|forwarded]] to hosts behind the gateway.
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h3. IKEv2 (Windows 7/8, Linux, Android 4+, Mac OS X, iOS 8+)
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The gateway configurations shown in the [[Windows7|Windows 7 section]] of the wiki may be used for all IKEv2 clients. In both use
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cases presented there the gateway is authenticated with a certificate, while the clients will either authenticate themselves
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with certificates, or use username and password. Both configurations may be implemented on a gateway to leave it to the
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clients to select an authentication method.
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With the [[EAPRadius|eap-radius plugin]] the user authentication may be delegated to a RADIUS server (e.g. an existing Active Directory DC).
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Both the [[AndroidVPNClient|strongSwan VPN Client for Android 4 and newer]] and the [[NetworkManager|strongSwan NetworkManager plugin]] may be
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used with either of these configs.
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For Linux roadwarriors that don't want or can't use the NetworkManager plugin [[IKEv2ClientConfig|this client config may be used]]. 
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Alternatively, the [[charon-cmd]] command line IKE client provides a simple means to establish roadwarrior connections since version:5.1.0.
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[[MacOSX#Native-application|Our app for Mac OS X]] supports IKEv2 and simple EAP authentication.
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With [[AppleIKEv2Profile|iOS 8 and Mac OS X 10.10]] (Yosemite) Apple introduced support for IKEv2 in their clients. A GUI to configure such connections
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is currently not provided, so it's necessary to write (or generate) [[AppleIKEv2Profile|custom configuration profiles]].
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h3. IKEv1 (iOS, Mac OS X, Android, Windows)
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The configuration presented on the [[iOS_(Apple)|iOS and Mac OS X page]] should work for all IKEv1 clients that support XAuth.
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For Windows hosts before Windows 7 it is recommended to use a third-party IPsec client like "Shrew": instead of the
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built-in IKEv1/L2TP client.
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Instead of generating a private key and certificate pair for each client you may also use the same key/certificate pair for
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all clients. The actual client authentication will then be based on XAuth (this is similar to hybrid authentication, but also works
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for clients that don't support it, or implement it incorrectly, like some iOS versions did). Even though the private key/certificate
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pair is "public", this still ensures proper authentication of the gateway, but might simplify deployment to clients.
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XAuth with PSK may also be used (see #218) but is not recommended for larger deployments.
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The XAuth credentials provided by the clients may be verified against the same RADIUS server used for IKEv2 clients with
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the help of the [[XAuthEAP|xauth-eap plugin]].
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h2. Site-to-Site Configurations
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For site-to-site connections you may refer to any of the _net2net_ scenarios (and many others) of [[ConfigurationExamples|our test suite]].
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The most important difference compared to the remote access case is that the initiator will not request a [[VirtualIP|virtual IP address]]
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but instead use _leftsubnet_ to tunnel traffic from one or more local subnets.  For IKEv2 multiple subnets (in CIDR notation) can
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be added to _left|rightsubnet_, separated by commas. If IKEv1 is used a separate [[ConnSection|conn section]] has to be added for each
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combination of left and right subnet as only the first subnet in _left|rightsubnet_ will be used (using either _conn %default_ or the
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_also_ keyword can reduce each of these configs to a few lines).
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One thing that often confuses users new to IPsec is that testing a net-to-net scenario from either of the gateways often requires
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one to select the source address used specifically (e.g. with @ping -I@) because the external IP of either gateway might not be
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included in the tunneled subnets. If that is something you require either add the external IPs to the list of subnets in _left|rightsubnet_
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or add a specific host-to-host config.
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h2. Host-to-Host Configurations
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Host-to-host connections are very easy to setup. You basically have to configure _right_ to the hostname or IP address of the peer
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and configure the desired authentication, neither leftsubnet nor rightsubnet have to be set explicitly.
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Again, [[ConfigurationExamples|our test suite]] provides several examples.